Apparatuses that perform a departure prevention process for vehicles are well known. In such a departure prevention process, a road surface image is acquired by an in-vehicle camera mounted to the vehicle (own vehicle) and the image is subjected to an image recognition process to detect a travel lane line on which the own vehicle travels. Then, based on the detected travel lane line, and the speed and yaw rate of the own vehicle, a driving determination process is performed, followed by performing the departure prevention process for the own vehicle according to the determination results (refer to PTL 1). In the departure prevention process, when it is determined, as a result of the driving determination process, that the own vehicle has a high probability of departing from a travel lane, an alarm is issued by sounding a buzzer, displaying an image, or the like, or the own vehicle is steered in a direction of preventing departure from the travel lane.